1. Field of the Invention
The present invention generally relates to optical signaling beacons to indicate the locations of persons and/or objects and, more particularly, to arrangements for synchronizing the signaling operations of a plurality of such beacons.
2. Description of the Prior Art
Signaling beacons have been known for many years, particularly for purposes of navigation under adverse lighting conditions such as night or fog. Lighthouses are a particularly familiar example of signaling beacons used for such purposes over the last century and longer. In more recent times, a plurality of signaling beacons have been used together to indicate not only the location but the size and shape of an object, facilities or obstructions such as runways for aircraft, construction barriers, communication signaling towers and the like or even locations of personnel.
In these latter applications, however, it has been found desirable to provide a degree of common control to such beacons to enhance visibility of an object. In order to identify a light source as a signaling beacon and possibly to identify the object indicated, it is desirable that the light output be other than continuous and unchanging. For example, in regard to a single signaling beacon, a unique pattern of light flashing was used in lighthouses not only to distinguish the signaling beacon from other light sources, such as on other boats or on shore, but to identify particular lighthouses and distinguish them from each other. In applications utilizing a plurality of signaling beacons, synchronized flashing lights or particular or changing colors may be used. For example, flashing lights are often used in combination with constant, steady light beacons on communication towers and lights flashing in a sequence indicative of direction are often used for airport runways. The cooperative operation of plural beacons in such a manner not only unambiguously identifies the object but facilitates the visualization of its location, size, orientation and configuration. Unfortunately, large, complex and generally expensive systems are required to achieve such effects and are generally applicable only to fixed installations with a predetermined number of regularly spaced beacons to be synchronized.
On the other hand, there are numerous applications for signaling beacons which are temporary and where the number and arrangement of beacons is arbitrary and/or changeable such as in the case of highway construction barriers as alluded to above. In many such cases, it is important that the hardware used be very simple, rugged and inexpensive due to the potentially large number of beacons required, the need for quick and simple installation by personnel not trained for sophisticated electrical or electronic installations and the relatively high likelihood of damage to or theft of individual signaling beacons. Therefore, there is generally no provision for synchronization of signaling beacons of this type. The resultant free-running flashing, even if accurately clocked at each beacon, tends to result in flashing in a substantially random order; yielding a “twinkling” effect overall which tends to inhibit the ability to easily and accurately visualize the pattern being marked by the signaling beacons or to identify the individual beacons as related in function or purpose. Moreover, in applications such as highway construction barriers where the array of signaling beacons is generally linear and first seen from a vantage point substantially colinear with the array, the blinking or flashing of individual signaling beacons may be obscured and inhibit recognition of the light as that of signaling beacons.
Attempts have been made to provide synchronization of such separated and free-running signaling beacons by accurate clocking and initial synchronization. However, such arrangements generally require the individual units to be brought into proximity with a given signaling beacon such that a synchronization connection or other physical contact can be made. Of course, this must be done for each signaling beacon to be deployed which presents a substantial inconvenience, particularly when it is considered that the time required to establish synchronization is, for simple systems, approximately equal to the cycle time of the beacon (e.g. often eight seconds or more). Moreover, after initial synchronization, the individual signaling beacons will operate in a free-running manner and, since the clocking arrangement is necessarily subject to drift, synchronization will invariably be lost and the twinkling or sparkling effect will be developed over a greater or lesser period of time depending on clock accuracy and stability; both of which are usually associated with increased cost.
Attempts to provide synchronization by gating the individual beacons with the detected light output of other signaling beacons has been generally complex and ineffective because of, among many possible reasons, the high likelihood that the observation or monitoring of other beacons may be ambiguous or obscured or subject to delays inherent in developing full light output or extinguishing it as the signaling beacon light is turned on and off as well as circuit delays and which may be cumulative across the system where synchronization control is serial from one beacon to the next. Such systems are also subject to interference from other light sources and distinguishing system light sources from other light sources such as by modulating the light output in accordance with a unique protocol is difficult. Such attempts at synchronization also require reliable and continuous communication links (e.g. line-of-sight) which may be very difficult if not impossible to reliably maintain, particularly in regard to personnel or movable objects.
It should be appreciated that slight deviation from synchronization (e.g. at the onset of the sparkling or twinkling effect and possibly before the difference in timing becomes evident in systems providing for initial synchronization) detracts significantly from the effect of a plurality of signaling beacons and perception of the object(s) or personnel which they mark or indicate. That is, any slight lack of uniformity or homogeneity in operation of beacons in a system is highly perceptible and tends to be perceived as different systems indicating application to different objects or personnel. This loss of uniformity is particularly critical in regard to indicating locations of personnel or objects which are movable, such as vehicles, aircraft or vessels.
Conversely, a perception of uniformity of operation of a plurality of signaling beacons is critical to recognition of the plurality of signaling beacons of arbitrary number and arrangement as constituting a single system or application. This is particularly true where individual signaling beacons are used to identify individual personnel or movable objects which must be distinguished from other personnel or movable objects (e.g. friend or foe discrimination) where it may even be attempted to emulate one system with another system. Uniformity of operation also assists in the perception of the array of beacons in applications such as route or boundary markers where the shape of the array is important, especially when the signaling beacons are operated with short duty cycle and/or long cycle times.